The present invention is related to liquid crystal display devices, and, in particular to active matrix liquid crystal display devices.
A liquid crystal display device changes the direction of orientation of liquid crystals by applying an electric field to a layer of liquid crystals held in between two substrates, and carries out display using the changes in the optical characteristics of the liquid crystal layer resulting from such changes in the direction of orientation of the crystals. In a conventional active matrix type liquid crystal display, as is typified by the twisted nematic (TN) display mode in which the display is made utilizing the optical rotation characteristics of liquid crystals, the orientation of the electric field is applied to the liquid crystal was set to be almost perpendicular to the substrate boundary surface. On the other hand, the method of carrying out display using the birefringence characteristics of liquid crystals (the in-plane switching mode) by making the orientation of the electric field applied using comb-tooth shaped electrodes to the liquid crystal almost parallel to the substrate, has been proposed, for example, in Japanese Patent Publication No. Sho 63-21907, and Japanese Unexamined Patent Publication No. 505247. This in- plane switching mode has the advantage of wider viewing angles compared to the conventional TN mode and is a very promising technology for active matrix type liquid crystal display devices. As liquid crystal materials for active matrix type liquid crystal display devices of the in-plane switching mode, it has been proposed to use liquid crystal mixtures having relatively low specific resistances (Japanese Unexamined Patent Publication No. Hei 7-306417), liquid crystal mixtures containing 4-(cyclohexylcarbonyloxy)-benzonitrile in order to achieve both low driving voltage and high response speed (Japanese Unexamined Patent Publication No. Hei 9-125063), liquid crystal mixtures containing chemical compounds having fluorine as a polar group (Japanese Unexamined Patent Publication Nos. Hei 9-85541 and Hei 9-181823), or liquid crystal materials containing constituents with cyano group (D. Klement et al, SID International Symposium 98, 26.3), etc.
Further, in this in-plane switching mode, the relationships given by the following equations [Eqn. 1] and [Eqn. 2] are known to exist between the driving voltage, the liquid crystal response time, and the physical properties of the liquid crystal material (Masahito Oh-e and Katsumi Kondo, Applied Physics Letters, Vol. 67, pp. 3895–3897, 1995; Masahito Oh-e and Katsumi Kondo, Applied Physics Letters, Vol. 69, pp. 623–625, 1996).τoff∝γ1×d2/K22  [Eqn. 1]Vth∝(L×√{square root over ((K22/Δε))})/d  [Eqn. 2]
Here, Vth is the threshold voltage of the liquid crystal, K22 is the twist elastic constant of the liquid crystal material, Δε is the dielectric anisotropy, L is the electrode spacing (see FIG. 1), d is the thickness of the liquid crystal layer (see FIG. 1) τoff is the response time of the liquid crystals from the voltage applied condition to the no-voltage condition, and γ1 is the rotational viscosity of the liquid crystals.
Further, it is possible to transform [Eqn. 1] and [Eqn. 2] respectively into [Eqn. 3] and [Eqn. 4] because d×Δn is almost constant in order to maintain the optical characteristics.τoff∝γ1/(K22×Δn2)  [Eqn. 3]Vth∝L×Δn×√{square root over ((K22/Δε))}  [Eqn. 4]
As can be seen from these equations, the response time τoff becomes shorter as the viscosity of the liquid crystal γ1 becomes lower, and the driving voltage becomes lower as the dielectric anisotropy Δε becomes larger. However, in the case of most liquid crystal materials, there is an almost proportional relationship between the viscosity and the dielectric anisotropy Δε, that is, there is a trend that the viscosity is lower in liquid crystals with smaller Δε and the viscosity becomes higher as Δε becomes larger. This is because there is a trend that the dipole moments of high polar liquid crystal molecules which make Δε of mixtures large are large, and the intermolecular interaction between molecules is large in materials with large dipole moment, and consequently, the viscosity of the entire liquid crystal becomes large. Therefore, in the in-plane switching mode of display, there is a trade-off between the high-speed response characteristics of liquid crystals and low driving voltage. In other words, if a large amount of low polarity component with Δε≦1 and relatively low viscosity, that is, a so called neutral component, is added, although the viscosity gets reduced and a fast response can be achieved, the driving voltage also increases at the same time. Further, if a large amount of high polar component with large Δε is added, although the driving voltage can be reduced, the viscosity increases thereby making the response of the liquid crystal slower. Furthermore, not much has so far been proposed about the method of controlling the twist elastic constant K22 which is one additional parameter affecting the driving voltage and the response time.
On the other hand, in order to achieve high contrast, a number of technologies have been developed for placing the spacers to keep the spacing between the pair of substrates constant in a non-displaying region of the display device. For example, such methods have been proposed as described in Japanese Unexamined Patent Publication No. Hei 10-170928, Japanese Unexamined Patent Publication No. Hei 9-61828, Japanese Unexamined Patent Publication No. Hei 6-250194, Japanese Unexamined Patent Publication No. Hei 5-53121, Japanese Unexamined Patent Publication No. Hei 5-173147, Japanese Unexamined Patent Publication No. Hei 8-160433, Japanese Unexamined Patent Publication No. Hei 8-292426, and Japanese Unexamined Patent Publication No. Hei 7-325298, etc.